1. Field of the Invention
This invention relates to an acoustooptic device for diffracting polychromatic light, and more particularly to such a system for effectively producing a single output beam of each optical wavelength by using (1) an acoustooptic cell which exhibits an anisotropic diffraction mode and (2) properly chosen incident angles and acoustic frequency bandwidths.
2. Description of the Prior Art
It is well-known in the art that when an acoustic beam of wavelength .LAMBDA. interacts with a light beam of wavelength .lambda., the former acts as a diffraction grating of spacing .LAMBDA. which diffracts the light beam at an angle .phi. approximately by EQU .phi. = .lambda./2.LAMBDA. = .lambda.f/2V (1)
where V and f, respectively, are the acoustic velocity and frequency. Efficient Bragg diffraction occurs when the distance across the sound beam in the direction of the light path is greater than (.LAMBDA..sup.2 /.lambda.). The Bragg condition is satisfied when the incident angle .theta. is equal to .phi..
Since the diffraction angle depends on the acoustic frequency f, it is possible to vary the direction of diffracted light by changing f. If in equation (1), f is changed by an amount .DELTA.f, the angle of diffraction of the light beam will change by EQU .DELTA..phi.= .lambda./V.DELTA.f (2)
It can be shown that the number of spots N resolvable by the scanned beam is EQU N = (D/V) .DELTA.f = .tau..DELTA.f (3)
where D is the aperture of the incident light beam and .tau. is the access time, i.e., the time required for the acoustic wave to cross the aperture. The reader is referred to the PROCEEDINGS OF THE IEEE, Vol. 54, No. 10, 1966, page 1430 for a derivation of equation (3).
As stated hereinbefore, to obtain Bragg diffraction, the incident and diffracted beams should be symmetrical with respect to the acoustic wavefront. For a given incident angle and optical wavelength, the diffraction angle will be equal to the incident angle only when the acoustic frequency is at a specific value f o. If the direction of the diffracted beam is changed by changing the acoustic frequency from f.sub.o, the angle of incidence of the incident beam should also be changed to restore symmetry and maximum diffraction efficiency.
Commonly assigned,U.S. Pat. No. 3,783,185, which issued on Jan. 1, 1974 to R. A. Spaulding, discloses an acoustooptic modulator which produces a composite output beam comprising a plurality of diffracted collinear component beams of selected wavelengths. Light containing several wavelengths is impinged upon an acoustooptic cell. Electrical signals of different fixed frequencies are generated with amplitudes independently modulated in accordance with a source of color information. The electrical signals are applied to a transducer attached to the cell to generate waves having frequencies which correspond to the individual fixed frequencies in the composite electrical signal. The acoustic wave cause the light impinging thereon to be diffracted in a plurality of the spectra corresponding in number to the fixed frequency signals applied to the transducer. The frequencies of the electrical signals are chosen so as to produce a composite output beam comprising collinear diffracted component beams of selected wavelengths.
However, in the modulator of the Spaulding patent, the angles separating the incident light beams corresponding to each fixed acoustic frequency are small. Since each acoustic frequency will diffract the incident light of each beam with an efficiency inversely proportional to the angle that beam makes with the optimum incident angle for that frequency, there will exist high intensity "cross talk" resulting in multiple diffracted beams from each frequency impressed on the acoustooptic cell. In other words, if red, green and blue light is impinged on the cell at incident angles optimum for three fixed acoustic frequencies, each frequency will cause a different angle of diffraction for each color light, resulting in nine beams (three red, three green and three blue), only one beam of each color being within the composite output beam. While the three beams making up the composite beam will be of the highest intensity, the other six beams will be of objectionable intensity and have to be blocked from reaching the output target.
While the Spaulding acoustooptic cell is practical for a modulator, where the angle of diffraction of the composite beam is fixed, it is apparent that such a device could not be used as a deflector since the six light beams outside of the composite output beam would also be diffracted at varying angles if the acoustic frequencies were not fixed. To provide a scanning light beam comprising a plurality of optical wavelengths, Spaulding employs a slotted mask to block the unwanted beams. The composite beam which passes the mask is then deflected or scanned by conventional means such as by a rotating prism.
In 1967, it was predicted by R. W. Dixon that, when the incident light beam is made to propagate perpendicular to the optic axis of a uniaxial birefringent crystal, deviations will occur from isotropic Bragg diffraction. In an article entitled "Acoustic Diffraction of Light in Anisotropic Media" in the IEEE Journal of Quantum Electronics, QE-3, 85 (1967), he indicated that one such deviation was that the angle of incidence .theta. no longer was constrained to approximate the angle of diffraction .phi.. In addition, he pointed out that the angle-of-incidence versus acoustic-frequency characteristic of such deflectors using birefringent media exhibit (1) a decreasing incident angle .theta. with increasing acoustic frequency f for small values of , (2) an increasing incident angle .theta. with increasing acoustic frequency f for large values of f, and (3) an inflection point at a frequency f' whereat d.theta./df = 0.
FIG. 2 shows experimental results confirming Dixon's prediction, wherein for each of a plurality of acoustic frequencies, the incident angle was varied until the most efficient angle was found. Curve "A" of FIG. 2 is a plot of the most efficient incident angle for each acoustic frequency for light of .lambda. = 0.442 .mu.m. Note that the curve exhibits a minimum or inflection point at approximately .theta.' = 3.4.degree. and f' = 83 MHz.